Mothers Against Canada might have been inspired by similar organizations in real life.
For instance, The Parents Music Resource Center, who are responsible for the labeling of CDs and tapes that is still in effect today. This labeling results in censorship, because many outlets won't carry recordings that have strong labels on them.
Also, Action for Children's Television, who cleaned up and sanitized Saturday morning cartoons to the point where new cartoons have to be totally wimpy and insipid, and classic old Warner Brothers cartoons are butchered to smithereens to the point where they're no longer worth watching.
With both of those organizations, we ought to realize that the leaders were spending less time with their kids because they were dedicating so much of their time to crusading against the media. One of the most thought-provoking parts of the South Park movie was when Kyle's mom is making a "think of the children!" speech, while shooing away her son who wanted to talk to her.